A number of vibration damping devices, which are designed for installation between components making up a vibration transmission system in order to provide vibration damped linkage and/or vibration damped support to these components, are known in the prior art. Such vibration damping devices have a construction in which a first mounting fitting adapted to mount onto one of the components making up a vibration transmission system and a second mounting fitting adapted to mount onto the other component are linked by a main rubber elastic body. With the goal of further enhancing vibration damping capabilities, there have also been proposed vibration damping devices of fluid-filled design adapted to utilize flow action of a fluid filling the interior, and implementation of such devices in applications such as automotive engine mounts for example is a current topic of research. A fluid-filled vibration damping device has a pressure-receiving chamber whose wall is partially constituted by the main rubber elastic body, and an equilibrium chamber whose wall is partially constituted by a flexible film, these pressure-receiving and equilibrium chambers being filled with a non-compressible fluid, and the pressure-receiving and equilibrium chambers communicating with one another through an orifice passage.
It must be noted that where a fluid-filled vibration damping device is implemented in an engine mount of an automobile or the like, the frequency and amplitude of vibrations input to the device will vary according to factors such as vehicle driving conditions and engine speed. A problem with fluid-filled vibration damping devices, however, is that while vibration damping is excellent with respect to vibration of a frequency range to which the orifice passage has been tuned beforehand, it is more difficult to achieve effective vibration damping of input vibration of a frequency range which falls outside of the tuning frequency range. Particularly when the input vibration has a higher frequency than the tuning frequency, the orifice passage will become substantially obstructed due to antiresonance action, posing a risk that vibration damping capabilities will be diminished owing to a sharp rise in dynamic spring action constant.
To address this issue, there have been proposed, for example in Patent Documents 1 to 4 (JP-U-7-18046, JP-A-5-248480, JP-A-4-160246, and JP-A-61-66243), fluid-filled vibration damping devices having variable orifice passage tuning frequency so as to provide effective vibration damping of several types of vibration having different frequencies. In these Patent Documents, two orifice-defining components assembled in a relatively rotatable manner have formed therein an orifice passage that extends in the direction of relative rotation of the two orifice-defining components, such that the passage length of the orifice passage is varied through relative rotation of the two orifice-defining components. This makes it possible to vary the tuning frequency of the orifice passage, which is established by the ratio of passage length to passage cross sectional area.
However, in the fluid-filled mounts disclosed in Patent Documents 1 to 4, it is necessary to provide on the outside of the mount body an electric motor for actuating rotation of the orifice-defining components; and for the actuator shaft, which transmits rotational actuating force of the electric motor to the orifice-defining components which are situated in the interior of the mount body, to pass through the wall of the fluid chamber whose interior is filled with non-compressible fluid. Such a placement structure for the actuator shaft will entail problems such as difficulty in ensuring sealing of the wall of the fluid chamber in the section thereof penetrated by the rotary actuator shaft; complicated construction; and less reliable seals or lower durability of the mount itself. Moreover, the procedure for sealing non-compressible fluid in the fluid chamber tends to be difficult, leaving numerous issues that must be resolved before such a design could be practical.